Recently, UWB-based wireless communication which executes communication using pulse signals having a very narrow time width is being realized. In the UWB communication, pulse signals having a very narrow time width of 1 nanosecond to 2 nanoseconds are used, and the positions and phases of the pulse signals on the time axis are changed to transfer information. While the use of pulse signals having a very narrow time width of 1 nanosecond or less significantly widens the signal band where signals for UWB communication occupy to 500 MHz or greater, the modulation process itself becomes unnecessary, so that the spectral power density can be reduced. This achieves a high data transfer characteristic and high-precision distance measurement.
Therefore, a wireless communication system using the UWB can suppress the influence of noise low at the time of communication, and hence can eventually lead to cost reduction. In addition, the wireless communication system using the UWB can reduce the influences of various kinds of noise including a multipath.
However, the UWB-based wireless communication system has a difficulty in carrier sensing which is peculiar to the UWB communication. Accordingly, there is an increasing necessity of designing a system capable of detecting radio signals transmitted from a counterpart communication apparatus with a high reliability.
FIG. 1 shows an example of the format structure of packet data to be used in ordinary UWB-based wireless communication. The example of the format structure relates to a frame based on the standards of, for example, IEEE 802.11, IEEE 802.15.4 or the like. As shown in FIG. 1, frame data has an SFD (Start Frame Delimiter) 102 added to a preamble 101 for indicating the presence of the frame data, and a payload portion 103 added following the SFD 102. A gap of, for example, 12 bytes may be provided between frames.
Dummy information to prevent reception failure of frame data is written in the preamble 101. In the UWB-based wireless communication, in case of making the preamble 101, a plurality of regular codes of pulse signals having a very narrow time width are arranged, and after that a plurality of inverted sequences of the pulse signals are arranged.
The transmitted signal is acquired and evaluated by the preamble 101 first. When it is discriminated that a correct signal has been transmitted, communication is initiated in synchronism with the signal.
Because the preamble 101 has the pulse signals arranged regularly according to predetermined codes as mentioned above, a communication apparatus which has received the frame data added with the preamble 101 can capture signals, execute synchronization and channel prediction, and thus execute communication by detecting the regular sequence of pulse signals and codes thereof in the preamble 101. Other communication apparatuses excluding the counterpart communication apparatus which is currently in communication can easily determine whether the channel is occupied currently by acquiring information written in the preamble 101.
The SFD 102 has predetermined codes arranged therein and serves as a boundary between the preamble 101 and the payload portion 103.
The payload portion 103 is a real data portion generated by a user who performs communication using a communication apparatus. As shown in FIG. 2, while a signal which constitutes the payload portion 103 is partly added with a pulse sequence indicated by “S” in the diagram, an area where no real data is present is treated as an empty area. That is, the payload portion 103 is expressed by codes which are a combination of a pulse sequence indicated by “S” in the diagram and an empty area. The codes in FIG. 2 are merely an example, and there are various sequences of codes according to real data. That is, the payload portion 103 is structured to have a sequence of pulse signals and empty areas arranged irregularly.
Therefore, other communication apparatuses excluding the communication apparatus which is currently in communication can determine whether a channel is occupied by acquiring information written in the preamble 101 of frame data transmitted by the communication apparatus which is currently in communication. However, mere acquisition of information written in the payload portion 103 makes it difficult to discriminate whether a channel is occupied. When other communication apparatuses excluding the communication apparatus which is currently in communication determines that a channel is not occupied at a present time, the other communication apparatuses themselves may use the channel. Accordingly, a plurality of different communication apparatuses start communicating with one communication apparatus using the same channel, and interfere with one another.
That is, it is necessary to provide a situation where when data is transmitted from another communication apparatus to one communication apparatus, a further communication apparatus will not start data communication using the same channel.
Conventionally, as illustrated in Non-patent Document 1, for example, there has been proposed ALOHA which can ensure multiple access using dynamic codes to suppress intersignal interferences. However, channel identification by the ALOHA has a problem that it is limited by the characteristics of devices installed in a communication apparatus, such as the processing performance, memory capacity and power supply.
There is a technique disclosed which time-sequentially controls reception of accesses from other communication apparatuses in order to prevent data transmission by different communication apparatuses at the same time (see, for example, Non-patent Documents 2 and 3). However, the technique has a difficulty in achieving synchronization.
Non-patent Document 1: R. Merz, J. Widmer, J. Y. Le Boudec, and B. Radunovic “A Joint PHY/MAC Architecture for Low-Radiated Power TH-UWB Wireless Ad-Hoc Networks”, Wireless Communication and Mobile Computing Journal, Special issue on UWB communications. 5 (5): 567-580, 2005
Non-patent Document 2: Moe Z. Win and Robert A. Scholtz. “Ultra-wide bandwidth time-hopping spread-spectrum impulse radio for wireless multiple-access communications,” IEEE Transactions on Communications, 48 (4): 679-691, 2000.
Non-patent Document 3: M.-G. Di Benedetto, L. De Nardis and M. Junk etc, “(UWB)˜2: Uncoordinated, Wireless, Baseborn, medium access control for UWB communication networks,” Mobile Network and Applications, special issue on WLAN optimization at the MAC and Network levels, 2004.